Electro-optic properties of LiKSO4 and LiK1-xRbxSO4 crystals

1-x Rb_xSO₄ for x varying between 0 and 0.50 were investigated by an ac modulation method based upon the Sénarmont arrangement. The electro-optic coefficient r_c, and for the first time the coefficients r₄₁ and r₅₁ were determined with good accuracy in pure LiKSO₄ around room temperature at a wavelength of 633 nm. The frequency dependence of the electro-optic coefficient r_c was measured in the frequency range from 1 kHz up to 1 MHz. The values of the factor of merit n_e ³r_c of the mixed crystals LiK_1-xRb_xSO₄ were obtained for the first time, for the concentrations x=0.05, 0.10, 0.20, and 0.50. The electro-optic properties are shown to be affected by the random presence of the rubidium ions in this disordered system. Received: 16 March 1998/Revised version: 2 June 1998     

Material tensor parameters of LiNbO3 relevant for electro- and elasto-optics

The complete set of self-consistent parameters of nominally undoped LiNbO₃ crystals of congruent composition that describe the electro-optic, piezoelectric, elasto-optic, elastic, and dielectric response has been determined by numerically evaluating available measurements. The parameters were determined at room temperature and consist of the low-frequency clamped dielectric constants ε^S _ij, elastic stiffness constants at constant electric field C^E _ijkl, piezoelectric stress coefficients e_ijk, elasto-optic constants at constant electric field p^E _ijkl, and clamped electro-optic coefficients r^S _ijk. It is shown that the complete set is required for calculating the effective electro-optic coefficients and dielectric constants in photorefractive applications of LiNbO₃. Received: 4 January 2002 / Revised version: 1 February 2002 / Published online: 14 March 2002     

Frequency and wavelength dependencies of the electro-optic coefficients in SBN:60 single crystal

We report the dependence of the high and low-frequency values of the linear electro-optic (EO) coefficient r_c=r₃₃-(n_o/n_e)³r₁₃ of Sr_0.6Ba_0.4Nb₂O₆ (SBN:60) single crystal on the laser wavelength. The EO response has been carried out from dc to 150 MHz for four wavelengths ranging from 465 to 1321 nm. The clamped r^S and unclamped r^T coefficients have been determined and their dispersions as a function of wavelength have been established and discussed.     

Measurement of the electro-optic coefficients: description and comparison of the experimental techniques

This paper presents a review of experimental methods used to determine electro-optic (EO) coefficients in bulk materials. The techniques are based on the relations describing the electro-optic effect in one-beam-ellipsometric or two-beam-interferometric optical arrangements. The methods are classified according to the nature (dc or ac) of the electric field applied onto the sample (static or dynamic methods) and with respect of the measured physical quantity (voltage, phase retardation, light beam intensity, etc.). Theoretical analysis and considerations of practical interest are presented in the article with particular attention paid to the resulting error in the determination of the EO coefficient associated with each method. A complete analysis of advantages and drawbacks of methods is finally given as a function of different experimental parameters such as the accuracy or sensitivity, the complexity of the implementation, the signal and the physical quantity used to derive the EO coefficient. Measurements of the coefficient r _c as obtained by means of the various methods on the same congruent crystal of LiNbO₃, are used to support the comparison between techniques. Received: 14 October 1999 / Revised version: 4 January 2000 / Published online: 23 February 2000     

Measurement of the electric form factor of the neutron at Q2 = 0.3-0.8 (GeV/ c) 2 ⋆

The electric form factor of the neutron, G_E,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q ² = 0.3, 0.6 and 0.8 (GeV/c)². Corrections for nuclear binding effects have been applied.This revised version was published online in March 2005. In the previous version, the email address of one author was inadvertently assigned to multiple authors.     

The electro-optic effect in proustite

The low-frequency unclamped electro-optic coefficients of proustite have been measured at an optical wavelength of 0.6328m. The results show thatr ₂₂ is comparable to that of lithium niobate,r ₁₃ andr ₃₃ are considerably lower. The maximum theoretical value is calculated at 10.8×10^–12 m/V about three times the largest measured coefficient value.     

A holographic method for the determination of all linear electrooptic coefficients applied to Ce-doped strontium-barium-niobate

Using c- and a-cut plates of strontium-barium-niobate doped with 0.66 mol.% cerium all nonzero components of the linear electrooptic tensor r₁₃, r₄₂, r₃₃ including the sign are determined from angular dependence of the two-beam coupling coefficient Γ using transmission and reflection geometry of holographic recording. We found: r₁₃=56 pm/V, r₄₂=38 pm/V, and r₃₃=333 pm/V. In addition we calculated the effective trap density N_eff to be 2.2×10¹⁷ cm^-3. The advantages of this method as well as the values obtained are discussed. Received: 19 November 1998 / Revised version: 15 January 1999 / Published online: 7 April 1999     

Electric and magnetic losses and gains in determining the sign of refractive index

Within the framework of classic electromagnetic theories, we have studied the sign of refractive index of optical medias with the emphases on the roles of the electric and magnetic losses and gains. Starting from the Maxwell equations for an isotropic and homogeneous media, we have derived the general form of the complex refractive index and its relation with the complex electric permittivity and magnetic permeability, i.e. , in which the intrinsic electric and magnetic losses and gains are included as the imaginary parts of the complex permittivity and permeability, respectively, as  = _r + i_i and μ = μ_r + iμ_i. The electric and magnetic losses are present in all passive materials, which correspond, respectively, to the positive imaginary permittivity and permeability _i > 0 and μ_i > 0. The electric and magnetic gains are present in materials where external pumping sources enable the light to be amplified instead of attenuated, which correspond, respectively, to the negative imaginary permittivity and permeability _i < 0 and μ_i < 0. We have analyzed and determined uniquely the sign of the refractive index, for all possible combinations of the four parameters _r, μ_r, _i, and μ_i, in light of the relativistic causality. A causal solution requires that the wave impedance be positive Re{Z} > 0. We illustrate the results for all cases in tables of the sign of refractive index. One of the most important messages from the sign tables is that, apart from the well-known case where simultaneously  < 0 and μ < 0, there are other possibilities for the refractive index to be negative n < 0, for example, for _r < 0, μ_r > 0, _i > 0, and μ_i > 0, the refractive index is negative n < 0 provided μ_i/_i > μ_r/_r.     

Electro-optic properties in undoped and Cr-doped LiNbO3 crystals

3 along the ferroelectric c axis in pure and chromium-doped crystals. We have studied four series of samples with various crystal compositions and doping concentrations to separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of chromium. In pure and slightly doped crystals, the electro-optic coefficient r_c shows a non-monotonous dependence on the controlled crystal composition, which reflects a similar behaviour of the corresponding dielectric permittivity. In the highly Cr-doped crystals r_c displays a large decrease with increasing Cr concentration whatever the crystal composition. The results have been discussed by means of the LiNbO₃ structure and the substitution site dynamics. Received: 14 November 1997/Accepted: 2 January 1998     

Correlation between the Soret coefficient and the static structure factor in a polymer blend

Mutual mass diffusion and thermal diffusion has been investigated in poly(dimethylsiloxane)/ poly(ethylmethylsiloxane) (PDMS/PEMS) polymer blends of equal weight fractions. Molar masses ranged from below 1 to over 20 kg/mol. Both the mutual mass (D) and the thermal diffusion (D_T) coefficient contain a thermally activated factor with an activation temperature of 1415 K. The molar mass dependence of D_T is due to an end-group effect of the local friction coefficient. The thermal diffusion coefficient in the limit of long chains and infinite temperature is D_T^0, = - 1.69×10^-7cm²(sK)^-1. The Soret coefficient S_T of blends far enough away from a critical point is proportional to the static structure factor S(q = 0).     

Second-order nonlinear optical properties of poled coumaromethacrylate copolymers

Second-order nonlinear optical properties of newly designed and synthesized coumaromethacrylate side-chain polymers are reported. The optimum poling conditions were determined experimentally. The optimum poling temperature for these side-chain polymers is well above the glass transition temperature. The second harmonic coefficient of films poled by coronaonset at elevated temperature and the linear electro-optic coefficient of films poled by contact electrodes were measured. The stabilized value of the second harmonic coefficient, d ₃₃, at 1064nm fundamental wavelength was found to be 13 pm/V. The linear electro-optic coefficient, r ₃₃, exhibits strong dispersion ranging from 2 to 12pm/V in the wavelength range 477 to 1115 nm.     

Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

MD simulation of concentrated polymer solutions: Structural relaxation near the glass transition

We examine by molecular dynamics simulations the relaxation of polymer-solvent mixtures close to the glass transition. The simulations employ a coarse-grained model in which polymers are represented by bead-spring chains and solvent particles by monomers. The interaction parameters between polymer and solvent are adjusted such that mixing is favored. We find that the mixtures have one glass transition temperature T _g or critical temperature T _c of mode-coupling theory (MCT). Both T _g and T _c (> T _g decrease with increasing solvent concentration . The decrease is linear for the concentrations studied (up to = 25%. Above T _c we explore the structure and relaxation of the polymer-solvent mixtures on cooling. We find that, if the polymer solution is compared to the pure polymer melt at the same T, local spatial correlations on the length scale of the first peak of the static structure factor S(q) are reduced. This difference between melt and solution is largely removed when comparing the S(q) of both systems at similar distance to the respective T _c. Near T _c we investigate dynamic correlation functions, such as the incoherent intermediate scattering function (t), mean-square displacements of the monomers and solvent particles, two non-Gaussian parameters, and the probability distribution P(ln r;t) of the logarithm of single-particle displacements. In accordance with MCT we find, for instance, that (t) obeys the time-temperature superposition principle and has relaxation times which are compatible with a power law increase close (but not too close) to T _c. In divergence to MCT, however, the increase of depends on the wavelength q, small q values having weaker increase than large ones. This decoupling of local and large-length scale relaxation could be related to the emergence of dynamic heterogeneity at low T. In the time window of the relaxation an analysis of P(ln r;t) reveals a double-peak structure close to T _c. The first peak correponds to “slow” particles (monomer or solvent) which have not moved much farther than 10% of their diameter in time t, whereas the second occurs at distances of the order of the particle diameter. These “fast” particles have succeeded in leaving their nearest-neighbor cage in time t. The simulation thus demonstrates that large fluctuations in particle mobility accompany the final structural relaxation of the cold polymer solution in the vicinity of the extrapolated T _c.     

Efficient ultraviolet photorefraction in LiNbO3

A nominally undoped LiNbO₃ crystal with a slightly broadened absorption edge is used to study beam coupling effects in the UV at 351 nm. At this wavelength the crystal exhibits a diffusion-dominated charge transport mechanism, which allows steady state beam amplification of up to 700 times, comparable to BaTiO₃ in the visible. The used crystal material was characterized by an absorption coefficient =2.68 cm^–1 at 351 nm and a maximal gain coefficient =13.94 cm^–1. This high gain value in the UV can be attributed to a hole diffusion-dominated charge transport mechanism together with a low bulk photovoltaic effect. We measured photovoltaic fields of the order of 550 V/cm.     

Influence of niobium doping on the electrical properties of 0.58Pb(Sc1/2Nb1/2)O3-0.42PbTiO3 single crystal

Piezoelectric single crystals of 0.58Pb(Sc_1/2Nb_1/2)-0.42PbTiO₃ and Nb⁵⁺-doped PSN-PT have been grown using flux technique. It is believed that the addition of Nb⁵⁺ creates lead vacancy in order to compensate charge neutrality. The structural distortion that occured in the doped crystals has been revealed through broadening of some peaks in X-ray diffraction studies. Niobium content that increased from 0.50 to 1.00 mol% might have induced more defect dipoles associated with . This plays a significant role in improving the ferroelectric, dielectric and piezoelectric properties. Our observations clearly show an increase in the spontaneous polarization (P_r), dielectric constant at room temperature, degree of diffuseness and transition temperature (T_c) and also a decrease in coercive field. The reasons behind these enhanced electrical properties are discussed in detail.     

Modulation-free acetylene-stabilized lasers at 1542 nm using modulation transfer spectroscopy

We report frequency stabilization of diode lasers using modulation transfer spectroscopy of an acetylene transition (¹³C₂H₂, ₁+₃, P(16)) at 1542 nm. We realize modulation-free acetylene-stabilized lasers with a frequency stability of about 10^–11 and an absolute frequency accuracy of about 20 kHz.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Electrooptic properties of lithium niobate crystals for extremely high external electric fields

The electrooptic effect in lithium niobate crystals (LiNbO₃) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10^-3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s₁₃|≤2.3×10^-21 m²/V² is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. Received: 29 October 2002 / Revised version: 14 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: ml@uni-bonn.de     

Thermal wave electroacoustic calorimetry in a Si photovoltaic cell

Power dissipation in a p–n ⁺ Si solar cell has been monitored using the electroacoustic technique. Experimental results are described by a vector model which takes into account the different physical locations of the several heat-generating or heat-consuming processes. Analysis of experimental data based on this model allows separation of the different contributions to the power dissipated in the cell and a direct insight into the power loss mechanisms in the device.Symbols Used in the Text D _th thermal diffusion coefficient of the sample - EA electroacoustic - E _gap energy of the gap of the semiconductor - hv energy of the incident light - i _dark cell current density in the dark - i ₀ saturation current density of the device - l sample thickness - n quality factor of the device - P total power dissipated in the cell in PA experiments - PA photoacoustic - P _dark total power dissipated in the solar cell in the dark in EA experiments - P _{fs, inj}, P _{bs, inj} cooling of front and back surface of the cell by injected carriers - P _{fs, photo}, P _{bs, photo} heating of front and back surface of the cell by photogenerated carriers - P _inj power dissipated in the recombination of injected carriers - P _jc power absorbed in the junction cooling process - P _light total power dissipated in the illuminated solar cell in EA experiments - P _photo power dissipated by photogenerated carriers when drifting along the potential slope - P _therm power dissipated by thermalization of photocarriers in the conduction band - V amplitude of the modulated potential applied to the cell - , phase angles between vector contributions to the total dissipated power in the cell - E _c, E _v energy difference between the quasi-Fermi levels of the n(p) semiconductor and the bottom (top) of the conduction (valence) band - _th=(2D _th/)^1/2 thermal diffusion length of the sample - _d phase of EA signal in the dark - ₁ phase of EA signal with light - frequency of the modulated voltage applied to the cell.     

Upconversion in YAlO3:Er pumped at 800 nm

Fluorescence at 3 m and 545 nm in a YAlO₃:Er (30 at.%) crystal has been measured as a function of wavelength, intensity and polarization for the excitation in the 800 nm wavelength region. Emission at 545 nm is an indicator of losses for 3 m emission due to excited-state absorption and upconversion. The experimental results allow to determine the excitation mechanisms, the upconversion coefficient W ₂ and the optimum pump wavelengths to generate 3 m radiation.     

Saturation of ionization edge absorption by donors in germanium

A study of saturation of the absorption and photoconductivity of Sb and P donors in Ge for radiation of 90 m wavelength, i.e., of energy very closely above their ionization edges is presented at T=9.3 K. Under these conditions negligible heating by the excess radiation energy is expected, which provides a convenient opportunity to study the kinetics of photoionization and recombination. From these measurements we have determined the donor capture cross section of electrons at 9.3 K to be _c=(1.2±0.7)×10^–12cm^–2, and the relaxation time from the 2s to the ground state as ₂₁=(5.8±1.0)×10^–10s. The saturation intensity of the absorption coefficient is around three orders of magnitude higher than the saturation intensity of the photoconductivity. We explain the nonlinear photoconductivity by the Debye-Conwell dependence of the mobility on the number of photoionized donors and compensating acceptors.